Four-wire ohmmeter connector and ohmmeter using same

ABSTRACT

A four-wire ohmmeter connector includes a pair of elongated members spaced apart from each other by an interconnecting web. A pair of elongated contacts are mounted on forwardly projecting portions of each of the elongated members. An insulative housing surrounds the elongated members, contacts and web. The contacts mounted on one of the elongated members are connected through separate wires to a positive probe, and the contacts mounted on the other of the elongated members are connected through separate wires to a negative probe. The elongated members are inserted into respective terminal apertures of a four-wire ohmmeter. A pair of semi-cylindrical conductive sleeves are aligned with each of the apertures, and they make contact with and compress the respective contacts that are inserted into the aperture.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/818,632, filed Jun. 15, 2007. This application is incorporated byreference herein in its entirety and for all purposes.

TECHNICAL FIELD

This invention relates to test and measurement equipment, and morespecifically to a connector for a four-terminal resistance measurementdevice.

BACKGROUND OF THE INVENTION

Test and measurement instruments, such a volt/ohmmeters, have been incommon use for many years. Conventional volt/ohmmeters include a pair oftest terminals, designated positive (“+”) and negative (“−”), that areconnected to test points through respective wires or “leads.” Each leadgenerally has a respective plug at one end that plugs into one of thetest terminals of the volt/ohmmeter, and a test probe, alligator clip,or other structure that is adapted to make electrical contact with atest point.

For AC or DC voltage measurements, the leads are connected to respectivetest points, and the voltage between the test points is measured by theinstrument. For resistance measurements, the volt/ohmmeter passes acurrent between the two leads, and the voltage between the two leads ismeasured. The resistance in ohms is then determined as the ratio ofvoltage-to-current. In practice, the resistance is measured simply bymeasuring the voltage since the current is generally constant. Themeasured voltage, or, more correctly, the measured voltage calibrated inresistance, can then be read by a user on an analog pointer-type meteror on a digital display.

Conventional two-wire volt/ohmmeters are very satisfactory in mostapplications. However, under some circumstances, they do not providesufficiently accurate resistance measurements. For example, when theresistance to be measured is very low, the resistance of the leads inrelation to the resistance to be measured can be unacceptably large.Under these circumstances, a significant portion of the voltage betweenthe leads as measured by the instrument can result from voltage drops inthe leads rather than across the resistance to be measured. Theinstrument will then provide an erroneous resistance measurement. Forexample, assume that the resistance to be measured is actually 0.5 ohms,and each lead has a resistance of 0.2 ohms. If the volt/ohmmetersupplies a current through the leads of 100 mA (i.e., 0.1 amp), thevoltage between the two leads as measured by the instrument would be0.09 volts (i.e., 0.1*0.9). The instrument would then calculate theresistance to be measured at 0.9 ohms (i.e., 0.09/0.1) when, in fact theresistance to be measured is really 0.5 ohms.

To alleviate this problem, four-wire ohmmeters have been developed. In afour-wire ohmmeter, each of two test points is connected to theinstrument through two separate leads. One of the leads in each paircarries the current to or from the resistance to be measured, and theother lead in the pair measures the voltage on one side of theresistance to be measured. Any voltage drop in the lead carrying thecurrent is not reflected in the reading since the voltage-measuring leadmeasures the voltage at the resistance to be measured rather than at thejunction between the instrument and the current-carrying lead. And sincethe input impedance of the resistance measuring circuit is generallyvery high, very little current is carried by the voltage-measuring lead,thus making any voltage drop across the lead nominal. An example of afour-wire ohmmeter is described in U.S. Pat. No. 5,508,621 to Wong,which is incorporated herein by reference.

Insofar as four-wire ohmmeters have four separate leads, the ohmmeteritself must have four test terminals. These test terminals are commonlyknown as the high (“HI”) terminal, the Sense HI terminal, which isconnected to the same test point as the HI terminal, the low (“LO”)terminal, and the Sense LO terminal, which is connected to the LOterminal. The HI terminal is generally connected to a current source,and the LO terminal is generally connected to a corresponding currentsink. The Sense HI terminal is connected to the positive input of avoltage measuring device, and the Sense LO terminal is connected to thenegative input of the voltage measuring device. These test terminalshave typically been in the form of cylindrical apertures or “jacks” thatare adapted to receive respective “banana plug” connectors. Thus fourbanana plug jacks have typically been required in a four-wire ohmmeter.

One approach to reducing the number of jacks in a four-wire ohmmeter isdescribed in the previously mentioned patent to Wong. The ohmmeterdescribed in the Wong patent uses two banana plugs physically connectedto each other through an insulator. Each of the banana plugs has twoaxially spaced contacts, which are connected to respective leads. Thetwo-contact banana plugs are inserted into respective jacks in theohmmeter, and each jack includes a pair of axially spaced contacts thatmate with respective contacts of the banana plug.

The connector shown in the Wong patent has the advantage of reducing thenumber of jacks required by the ohmmeter, and it is also structured tobe compatible with conventional two-wire leads, albeit without thebenefit of a four-wire measurement. Although this connector represents asignificant improvement in the art, it is nevertheless less than idealin some situations. For example, insofar as the contacts of the bananaplug are exposed, their contact surfaces can more easily becomecontaminated, which may undesirably increase their connectionresistance. Also, it is possible for the connector shown in the Wongpatent to make incorrect connections to the ohmmeter. For example, ifthe banana plugs are not inserted a sufficient distance into theirrespective jacks, the end contact of the banana plug may bridge theinner and outer contacts of the jack. In such case, the ohmmeter wouldprovide a resistance measurement, but it would not be apparent that themeasurement being made is essentially using only two rather than fourwires.

There is therefore a need for an improved connector for use with afour-wire ohmmeter.

SUMMARY OF THE INVENTION

A four-wire test connector includes first and second, electricallyinsulative elongated members separated from each other a predetermineddistance by an interconnecting web. A pair of elongated electricalcontacts are mounted on opposite surfaces of each of the elongatedmembers and are spaced apart from each other so they are electricallyisolated from each other. The elongated members and interconnecting webare preferably surrounded by a housing, which may include an insulativecylindrical member co-axially surrounding each of the elongated members.The contacts are preferably formed of a resilient material, and they bowoutwardly at their midsections. The contacts mounted on one of theelongated members are preferably connected through first and secondconductive wires to a positive probe, and the contacts mounted on theother of the elongated members are preferably connected through thirdand fourth conductive wires to a negative probe. In use, the elongatedmembers are preferably inserted into respective terminal apertures of afour-wire ohmmeter, and the contacts on each elongated member arecompressed by a pair of semi-cylindrical conductive sleeves, whichconnect the contacts to internal circuitry in the ohmmeter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded isometric view of a four-wire ohmmeter connectoraccording to one example of the invention.

FIG. 2 is a partially-assembled isometric view of the four-wire ohmmeterconnector of FIG. 1.

FIG. 3 is a fully-assembled isometric view of the four-wire ohmmeterconnector of FIGS. 1 and 2.

FIG. 4 is an exploded isometric view of a four-wire ohmmeter connectoraccording to another example of the invention.

FIG. 5 is an isometric view of a four-wire ohmmeter being used tomeasure resistance through 4 wires, which are connected to the ohmmeterusing the connector of FIGS. 1-3.

DETAILED DESCRIPTION

FIGS. 1-3 show one example of a four-wire ohmmeter connector 10. Withreference to FIG. 1, the connector 10 includes a frame 12 formed withtwo elongated members 14, 16 interconnected by a web 18 at one end sothat the elongated members 14, 16 are spaced apart from each other by apredetermined distance and project forwardly from the web 18. In oneexample of the frame 12, the centerlines of the elongated members 14, 16are spaced-apart from each other by 0.75 inches. The frame 12 isfabricated of a suitable material, such as plastic. The material formingat least the elongated members 14, 16 should also be electricallyinsulating. The web 18 is preferably but not necessarily formed with aseries of apertures 20, which reduce the weight of the frame 12 and theamount of material required to form the frame 12 without undulydecreasing the strength of the frame 12. Each of the elongated members14, 16 has generally flat upper and lower surfaces 22 on whichspaced-apart clamping members 24 and a rectangular recess 26 are formedfor reasons that will be explained below. The clamping members 24 andrectangular recesses 26 are formed on both the upper surfaces of theelongated members 14, 16 (as shown in FIGS. 1 and 2), and the lowersurfaces of the elongated members 14, 16 (which are not shown in FIGS. 1and 2). Finally, a partially spherical member 28 is formed at theforwardly-projecting end of each of the elongated members 14, 16.Although not shown in FIG. 1, a pair of slots 30 are formed along therear surface of each of the partially spherical members 28, which aresubstantially aligned with the flat upper and lower surfaces 22,respectively.

Each of the elongated members 14, 16 carry a pair of contacts 40 ontheir upper and lower surfaces. The contacts 40 each include a somewhatcylindrical portion 42 having a diameter that increases from each endtoward its midsection where a constant diameter portion 44 is formed. Asa result, the contacts 40 bow outwardly at their midsections. Thecontacts 40 are preferably formed of a suitable material, such astellurium copper or gold plated beryllium copper, to provide goodresiliency and conductivity. However, in some embodiments, the contacts40 need not be resilient. An elongated slot 48 is formed in eachcylindrical portion 42 so that the cylindrical portion 42 can becompressed, as explained in greater detail below. A tab 50 projects fromthe end of each contact 40. The tab 50 fits into the slot 30 formed inthe partially spherical member 28 to secure the contacts 40 to the frame12, as shown in FIG. 2. A pair of crimping members 54 are formed in eachcontact 40 at the end opposite the tab 50. In the example of theconnector 10 shown in FIG. 1, the contacts 40 are positioned onsymmetrically opposite sides of the web 18, i.e., the tops and bottomsof the elongated members 14, 16. However, it should be understood thatthe contacts 40 could have a different orientation, but a differentorientation may be less desirable because it could limit the lengths ofthe elongated members 14, 16 and/or make it more difficult to connectthe contacts 40 to respective wires, as described below.

With further reference to FIG. 2, when the contacts 40 are secured tothe frame 12, the crimping members 54 fit into the rectangular recesses26 (FIG. 1) formed on the flat surfaces 22, and the crimping members 54are aligned with the clamping members 24. A respective wire 60 has aconductor 64 crimped between the crimping members 24 of each contact 40,and an insulative coating 68 held by the clamping members 24. Theclamping of the insulative coating 68 by the clamping members 24 and thecrimping of the conductor 64 by the crimping members 24 has the effectof securing the rear portion of each conductor 40 to the frame 12.

After the contacts 40 have been secured to the frame 12 and the wires 60have been secured as sown in FIG. 2, a housing 62 is secured around theframe 12 and contacts 40 as shown in FIG. 3. The housing 62 is formed byany suitable material, such as plastic. The housing 62 includes a pairof cylindrical portions 64 that surround the forwardly extendingportions of the elongated members 14, 16, including the contacts 40. Thecylindrical portions 64 perform the function of electrically isolatingthe contacts 40 from the external environment so the surfaces of thecontacts 40 do not become contaminated, which might unduly increasetheir surface resistance. The housing 62 also includes a pair ofcylindrical portions 66 having a diameter larger than the portions 64,which are interconnected by a intermediate portion 68 that surrounds theweb 18 of the frame 12. Finally, a pair of conical portions 70 projectrearwardly to an opening through which respective ones of the wires 60extend.

FIG. 4 shows another example of a four-wire ohmmeter connector 10′. Theconnector 10′ includes substantially the same components that are usedin the connector 10 shown in FIGS. 1-3. Therefore, in the interest ofbrevity, the components have been provided with corresponding referencenumbers, and a description of their structure and operation will not berepeated. The connector 10′ differs from the connector 10 shown in FIGS.1-3 primarily in the shape of its components. For example, the frame 12′is thicker than the frame 12 used in the connector 10, the elongatedmembers 14′, 16′ are more cylindrical than the elongated members 14, 16used in the connector 10, and the contacts 40′ are substantially thinnerand shaped differently compared to the contacts 40 used in the connector10.

The four-wire ohmmeter connector 10 or 10′ is shown in use with anohmmeter 80 of conventional design in FIG. 5. The ohmmeter 80 includes adigital display 84, manually operable controls 86, and a pair ofapertures 88. The connector 10 is plugged into the apertures 88 of theohmmeter 80, and two wires 60 a,b and 60 c,d extend from the openingformed at the end of the respective conical portions 70. As shown inFIG. 5, the wires 60 a,b are in the form of a first coaxial cable, andthe wires 60 c,d are in the form of a second coaxial cable. However,separate wires or other wire configurations may be used. The wires 60a,b are connected to a positive voltage probe 74 a, and the wires 60 c,dare connected to a positive current probe 74 b. The probes 74 a, b areplaced in electrical contact with terminals T of a resistance R to bemeasured. The probes 74 a-d may be of conventional design, such asalligator clips, needle probes, clip-on probes, and banana plugs, toname a few. The probes 74 a-d are marked with suitable designationsand/or color coding identifying their polarity as being either positiveor negative. Alternatively, the two wires 60 a,b used to sense positivevoltage and current may be connected to separate probes, and the twowires 60 c,d used to sense negative voltage and current may be connectedto separate probes.

The connector 10 is plugged into the ohmmeter 80 by inserting each ofthe cylindrical portions 64 into a respective one of the apertures 88.Aligned with each of the apertures 88 is a pair of electricallyconductive, semi-circular sleeves 90, which are electrically isolatedfrom each other. The outside diameter of the sleeves 90 is small enoughto fit within the cylindrical portions 64, and the inside diameter ofthe sleeves 90 is selected to surround the contacts 40 in a slightlycompressed condition. However, in some embodiments of the connector 10in which the contacts 40 are not be resilient, the sleeves 90 may beresiliently biased inwardly to maintain contact with the contacts 40.When each of the cylindrical portions 64 has been inserted through arespective one of the apertures 88, one of the contacts 40 mounted on arespective one of the elongated members 14, 16, makes contact with oneof the semicircular sleeves 90, and the other contact mounted on thesame elongated member 14, 16 makes contact with the other semicircularsleeve 90. Therefore, in the event the contacts 40 have an angularorientation in which they are not symmetrically positioned relative tothe web 18, the angular orientation of the sleeves 90 would be alteredso that each contact 40 makes electrical contact with one and only onesleeve 90. Insofar as the contacts 40 can touch the respective sleeves90 over a considerable distance along the lengths of the sleeves, theproper connections between the probes and the ohmmeter 80 of thecylindrical portions 64 are not inserted all of the way into therespective apertures 88.

Although, the sleeves 90 may be connected to internal circuitry 100 inthe ohmmeter 80 in a variety of ways, the sleeves 90 are preferablyconnected through the respective wires 60 to probes contacting the sameterminal T of the resistance R to be measured. In other words, onesleeve 90 aligned with one aperture 88 is preferably connected to thepositive voltage probe 74 a while the other sleeve 90 aligned with thesame aperture 88 is preferably connected to the positive current probe74 b. Similarly, one sleeve 90 aligned with the other aperture 88 ispreferably connected to the negative voltage probe 74 c while the othersleeve 90 aligned with the same aperture 88 is preferably connected tothe negative current probe 74 d. Connecting the sleeve 90 to theinternal circuitry 100 in this manner allows the connector to be fully“backward compatible.” Specifically, the four-wire ohmmeter 80 canaccept a suitably sized two-wire connector (not shown) and still provideaccurate resistance measurements since each wire will connect to thesleeves 88 having the same voltage and current polarity. In fact, thefour-wire connector 10 can be used as a two-wire connector by acceptingtwo wires (not shown), each of which is connected to either one or bothof the contacts 40 mounted on a respective one of the elongated members14, 16. Also, the connector 10 may be inserted into suitably sizedapertures of conventional two-wire ohmmeters (not shown), and thecorrect resistance measurement will be obtained when the probes areconnected in the same manner as with a four-wire resistance measurement.

Although the present invention has been described with reference to thedisclosed embodiments, persons skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. Such modifications are well within the skillof those ordinarily skilled in the art. Accordingly, the invention isnot limited except as by the appended claims.

1. A four-wire test connector, comprising: first and second,electrically insulative elongated members separated from each other by apredetermined distance by an interconnecting web; and a pair ofelongated electrical contacts mounted on opposite surfaces of each ofthe elongated members, the contacts mounted on each of the elongatedmembers being spaced apart from each other so they are electricallyisolated from each other.
 2. The four-wire test connector of claim 1,further comprising a housing surrounding the elongated members andinterconnecting web, the housing including an insulative cylindricalmember surrounding and co-axial with each of the elongated members. 3.The four-wire test connector of claim 1 wherein the elongated membersproject longitudinally from the web and each terminate in an enlargedportion in which a pair of slots are formed on opposite surfaces of theelongated member, and wherein a respective tab projects from one end ofeach of the contacts and is received by a respective one of the slots tosecure the contacts to the elongated members.
 4. The four-wire testconnector of claim 1 wherein the contacts are formed of a resilientmaterial and the contacts bow outwardly toward their midsections.
 5. Thefour-wire test connector of claim 4 wherein an elongated slot is formedin each contact to facilitate resilient compression of the contact. 6.The four-wire test connector of claim 1 wherein a pair of crimpingmembers are formed at one end of each of the contacts.
 7. The four-wiretest connector of claim 1, further comprising a conductive wireconnected to each of the contacts.
 8. The four-wire test connector ofclaim 7, wherein one of the wires connected to the contacts mounted onone elongated member are designated positive wires and the contactsmounted on the other elongated member are designated negative wires. 9.A four-wire test lead set, comprising: a positive probe and a negativeprobe; first and second conductive wires connected to the positiveprobe; third and fourth conductive wires connected to the negativeprobe; a four-wire connector, comprising: a first and second,electrically insulative elongated members separated from each other by apredetermined distance by an interconnecting web; a pair of elongatedelectrical contacts mounted on opposite surfaces of each of theelongated members, the contacts mounted on each of the elongated membersbeing spaced apart from each other, one of the electrical contacts beingconnected to the first conductive wire, another of the electricalcontacts being connected to the second conductive wire, another of theelectrical contacts being connected to the third conductive wire, andthe last of the electrical contacts being connected to the fourthconductive wire.
 10. The four-wire test lead set of claim 9 wherein thecontacts that are connected to the first and second conductive wires aremounted on one of the elongated members, and the third and fourthconductive wires are mounted on the other of the elongated members. 11.The four-wire test lead set of claim 9 wherein the first and secondconductive wires are connected to a common probe, and the third andfourth conductive wires are connected to a common probe, which isdifferent from the probe to which the first and second conductive wiresare connected.
 12. The four-wire test lead set of claim 9, furthercomprising a housing surrounding the elongated members andinterconnecting web, the housing including an insulative cylindricalmember surrounding and co-axial with each of the elongated members. 13.The four-wire test lead set of claim 9 wherein the elongated membersproject longitudinally from the web and each terminate in an enlargedportion in which a pair of slots are formed on opposite surfaces of theelongated member, and wherein a respective tab projects from one end ofeach of the contacts and is received by a respective one of the slots tosecure the contacts to the elongated members.
 14. The four-wire testlead set of claim 9 wherein the contacts are formed of a resilientmaterial and the contacts bow outwardly toward their midsections. 15.The four-wire test lead set of claim 14 wherein an elongated slot isformed in each contact to facilitate resilient compression of thecontact.
 16. The four-wire test lead set of claim 9 wherein a pair ofcrimping members are formed at one end of each of the contacts.
 17. Afour-wire ohmmeter, comprising: a housing; a display mounted in thehousing and being visible externally from the housing; electricalcircuitry mounted within the housing and connected to the display, theelectrical circuitry including a current source, a current sink, and avoltage measuring device having a positive terminal and a negativeterminal; and first and second apertures formed in the housing, each ofthe terminal apertures having a pair of semi-cylindrical conductivesleeves aligned with the respective aperture and electrically isolatedfrom each other, the sleeves being connected to the electrical circuitryin a manner so that one of the sleeves aligned with the first apertureis connected to the current source, the other of the sleeves alignedwith the first aperture is connected to the positive terminal of thevoltage measuring device, one of the sleeves aligned with the secondaperture is connected to the current sink, and the other of the sleevesaligned with the second aperture is connected to the negative terminalof the voltage measuring device.